Long-term Disconnect Between Nutrient Inputs and Riverine Exports in a Semi-arid, Agricultural Watershed: Yakima River Basin 1945-2012

Washington State University, 2020
Nutrient budgets are useful tools in addressing impaired water quality from increasing anthropogenic (N) and phosphorus (P) inputs to watersheds across the United States. Long term analyses in particular can identify temporal ecological responses to past nutrient management strategies. Multi-decade watershed analyses of nutrient dynamics traditionally have been limited to large temperate river basins focused primarily on a single nutrient (N or P). Here we present a long term, dual N and P input-output budget for the agriculturally rich Yakima River Basin (YRB) located in the semi-arid western US. We quantified N and P terrestrial inputs from 1945-2012 using past agricultural and water quality data in order to investigate interactions between water flow and N and P exports in the YRB. Between 1945 and 2012, incoming agricultural N and P increased by over 18 and 10-fold, respectively, with synthetic fertilizer accounting for the majority of inputs and increases. While such increases in YRB nutrients inputs were comparable to other US basins, observed riverine N and P export did not significantly increase over time. Rather, we found that since the mid-1970s, both N and P loads and concentrations in the Yakima River have decreased; possibly indicating a temporal disconnect between nutrient inputs and riverine exports in the YRB. This disconnect highlights a need to further investigate nutrient dynamics at local scales in semi-arid watersheds. Our analysis raises questions about long-term N and P retention in the YRB and potential impacts on future water quality across the western US.

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